Save/Export image from compositing tab in Blender 2.81
One way is to connect the Viewer Node.
Then just open your Image Editor, select Viewer Node and save the image.

You can save to any format you need .jpg/.EXR are all supported.



Note:
If the Viewer node doesn't show any image, just toggle one of your nodes off/on.
Viewer/Composite nodes refresh only when there's a change in the compositor.